1. Field of the Invention
The present invention relates to a support for a lithographic printing plate, and a presensitized plate, particularly, a thermosensitive presensitized plate suitably used for a computer to plate system requiring no development, more particularly, a thermosensitive presensitized plate, which can record images by infrared ray scanning exposure based on digital signals, and can carry out printing by being directly loaded on a printing machine without any execution of a conventional development process including use of developer and the like after exposure, and a support for a lithographic printing plate used for the same.
2. Description of the Related Arts
A number of studies have been conducted on presensitized plates for a computer to plate system, which have developed remarkably in recent years. Particularly among them, for the purpose of further streamlining a process and solving a problem of waste liquid disposal, many studies have been conducted and various ideas have been presented on a presensitized plate capable of carrying out printing by being directly loaded on a printing machine after exposure.
One of promising technologies may be a thermosensitive presensitized plate including a water receptive layer formed as an image forming thermosensitive layer, the water receptive layer containing hydrophobic thermoplastic polymer particles dispersed in hydrophilic binder polymer. This presensitized plate uses a principle that when heat is applied to the thermosensitive layer, hydrophobic thermoplastic polymer particles are fusion-bonded, and a surface of the water receptive thermosensitive layer is converted into an ink receptive image area.
One of methods for reducing processing steps in such a presensitized plate using the fusion bonding of hydrophobic thermoplastic polymer particles may be so-called on-machine development, which loads the presensitized plate after exposure to a cylinder of a printing machine without treating it in developer, and supplies ink and/or fountain solution while rotating the cylinder, thereby removing non-image areas of a recording layer of the presensitized plate. In this method, after the exposure, the presensitized plate is directly loaded on the printing machine, and development is completed in a normal printing process.
Such a presensitized plate suited to the on-machine development must have a thermosensitive layer soluble in fountain solution or ink solvent, and a lighted room handling characteristic suited to development on the printing machine placed in the lighted room.
For example, JP 2938397 B (the term “JP XXXXXXX B” as used herein means an “Japanese patent”) describes a presensitized plate including a thermosensitive layer formed on a water receptive support, the thermosensitive layer containing fine particles of thermoplastic hydrophobic polymer dispersed in hydrophilic binder polyer. In this specification, it is described that the presensitized plate is subjected to infrared laser exposure, the fine particles of thermoplastic hydrophobic polymer are combined by heat to form an image, then the plate is attached onto a printing machine cylinder and, by supplying ink and/or fountain solution, on-machine development can be carried out.
In addition, in JP 9-127683 A (the term “JP XX-XXXXXX A” as used herein means an “unexamined published Japanese patent application”) and WO 99/10186, it is described that after thermoplastic fine particles are combined by heat, on-machine development is carried out to manufacture a lithographic printing plate.
However, in the case of the presensitzed plate, which combines fine particles by heat to form images, problems have been inherent, including a low sensitivity caused by releasing of heat to a metal support and an insufficient press life caused by a low strength of the image area of the thermosensitive layer when the fine particles are not sufficiently combined while a good on-machine developing characteristic is exhibited.
As a countermeasure, a method of providing water insoluble organic polymer between an aluminum support and a thermosensitive layer has been presented (e.g., JP 2000-23983 A). However, this method has had a scumming problem while a sensitivity has been increased.
The following problems have been inherent in conventional presensitized paltes of thermal types which don't carry out on-machine development. Those plates include a positive presensitized plate of a so-called thermal type for causing photothermal conversion by infrared absorbent present in a thermosensitive layer, generating heat by exposure, and making the exposed portion of the thermosensitive layer alkali-soluble by the heat to form a positive image, and a negative presensitized plate of a thermal type for generating radicals or acids with radical generator or acid generator by the heat, thereby progressing radical polymerization reaction or acid crosslinking reaction to form a negative image.
That is, in the thermal type image forming, laser beam irradiation makes photothermal conversion material generate heat in the thermosensitive layer, and this heat causes image forming reaction. However, in an aluminum support which is grained and provided with an anodized layer, heat generated in the vicinity of an interface between the thermosensitive layer and the support is diffused inside the support before it is sufficiently used for image formation because heat conductivity of the support is extremely higher than a thermosensitive layer. Consequently, the following problems occur in the interface between the thermosensitive layer and the support.
First, in the thermosensitive layer of the positive working type, when alkali solubilization reaction isn't enough, heat is diffused inside the support and a problem of a low sensitivity causes, that is, residual layers are formed on an area to be a non-image area. This is a basic problem inherent in the positive working type thermosensitive layer.
In addition, in the presensitized plate of such a thermal positive working type, the use of infrared absorbent having a photothermal conversion function is essential. However, the absorbent has low solubility because of its relatively large molecular weight, and adsorbed on micropores formed by anodizing, and made difficult to be removed. Thus, residual layers are easily formed in the development process using alkali developer.
In the case of the negative working type thermosensitive layer, when heat is diffused inside the support, and developer insolubilization of the thermosensitive layer becomes insufficient in the vicinity of the interface between the thermosensitive layer and the support, an image is not sufficiently formed in an area to be an image area, disappearing during development, and even if an image is formed, the image area is easily peeled off during printing.
In order to solve the foregoing problems, attempts have been made to increase micropores on an anodized layer with a view to suppressing diffusion of heat generated on the thermosensitive layer in the aluminum support.
However, in the method of increasing micropores on the anodized layer, scum resistance is reduced while a sensitivity and a press life are increased. In addition, when a thermosensitive layer of an on-machine development type is provided, an on-machine development characteristic is also deteriorated.
From a similar perspective, attempts have been made to seal micropores by a method of dipping an aluminum support having an anodized layer formed on an aluminum plate in hot aqueous solution containing hot water, or inorganic or organic salt, a method of exposing it to steam bath, and the like.
However, in the method of sealing the micropores, a sensitivity and a press life are reduced while scum resistance is increased. In any case, therefore, it has not achieved a satisfactory level yet.
Under these circumstances, the inventors have presented a presensitized plate in Japanese Patent Application No. 2001-9871, which comprises an aluminum support including an anodized layer formed on an aluminum plate, and a particle layer containing particles having an average particle size of 8 to 800 nm and a recording layer recordable by infrared laser exposure formed in this order on the aluminum support. In this specification referenced herein, the inventors have also presented a method of providing the particle layer on the aluminum support by electrolyzing the aluminum support using electrolyte containing hydrophilic particles having an average particle size of 8 to 800 nm. According to this method, openings thereof can be sealed while voids are left inside micropores present on the anodized layer. Thus, it is possible to provide a presensitized plate high in sensitivity and press life, and also scum resistance can be increased.
However, regarding the increase in scum resistance, a level of increase achieved has not been satisfactory.